The present invention relates to improvements made with a view to enhancing vacuum in a very high vacuum system (ultrahigh vacuum), comprising a chamber which is capable of releasing gas at its surface.
In a metal system used as a heating chamber in which very high vacuum is generated (i.e. a vacuum of at least 10xe2x88x9210 Torr 10xe2x88x928 Pa), even in the order of 10xe2x88x9213 to 10xe2x88x9214 (10xe2x88x9211 to 10xe2x88x9212 Pa)), the metal walls of the chamber constitute an inexhaustible source of gas. The hydrogen contained in the metal structure (for example stainless steel, copper, aluminium alloy) is freely dispersed in the thickness of the metal and is released at the surface defining the chamber. Similarly, when the walls of the vacuum chamber are bombarded by particles (synchrotron, electron or ion radiation)xe2x80x94as is the case in particle acceleratorsxe2x80x94heavier molecular species are expelled, such as CO, CO2, CH4, produced at the surface after dissociation of hydrocarbons, carbides and oxides.
The vacuum level obtained in the chamber is therefore defined by the dynamic equilibrium between the release of gas at the surface defining the chamber and the pumping rate of the pumps used. Producing high vacuum involves a dual requirement of ensuring that the surface of the chamber is extremely clean so as to reduce the emission of gas and applying a high pumping rate. In the case of vacuum systems for particle accelerators, in which the chambers are generally small in section, either the pumps must be arranged close to one another or pumping has to be applied continuously in order to overcome the conductance limitation.
Under these conditions, a known approach to obtaining as high a vacuum as possible is to supplement the vacuum produced by mechanical pumps by applying complementary pumping, in particular by means of a getter arranged inside the chamber: this material is capable of producing chemically stable compounds by a reaction with the gases present in the vacuum chamber (in particular H2, O2, CO, CO2, N2) and this reaction causes the molecular species concerned to disappear, which is tantamount to a pumping effect.
However, regardless of the pumping process used and irrespective of the distributed efficiency that can be achieved by using a non-evaporable getter, the vacuum level likely to be obtained in the chamber is still defined by the dynamic equilibrium between the pumping rate (regardless of the means used) and the rate at which gas is released from the metal surface of the chamber (irrespective of the cause); in other words, the vacuum level remains dependent on the rate of release of gas inside the chamber for any given pumping rate.
In order to improve the quality of the ultrahigh vacuum inside the chamber, it is therefore desirable to try to reduce significantly the rate at which gases are released at the surface of the metal wall of the chamber and whilst doing to so increase significantly the efficiency of the pumping means.